This invention relates to protective coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) formed of a zirconia-based ceramic material that exhibits improved erosion and impact resistance as a result of containing a dispersion of alumina particles or precipitates.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to thermal damage and oxidation and hot corrosion attack, and may not retain adequate mechanical properties. For this reason, these components are often protected by a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating ceramic topcoat, typically referred to as the TBC. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element such as hafnium, zirconium, etc.), and diffusion coatings such as diffusion aluminides, notable examples of which are NiAl and NiAl(Pt).
Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation of the TBC. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). In contrast, plasma spraying techniques such as air plasma spraying (APS) deposit TBC material in the form of molten xe2x80x9csplats,xe2x80x9d resulting in a TBC characterized by flat (noncolumnar) grains and a degree of inhomogeneity and porosity that reduces heat transfer through the TBC.
While YSZ TBC""s are widely employed in the art for their desirable thermal and adhesion characteristics, they are susceptible to chemical and mechanical damage within the hot gas path of a gas turbine engine. In U.S. Pat. No. 4,996,117 to Chu et al., a YSZ TBC is disclosed whose individual grains are enveloped by a coating of zirconium silicate (zircon; ZrSiO4), silicon dioxide (silica; SiO2), aluminum oxide (alumina; Al2O3), aluminum silicate (SiO2/Al2O3) and/or aluminum titanate (Al2O3/TiO2) that protects the YSZ from corrosion, such as from attack by vanadium pentoxide. In terms of mechanical damage, YSZ coatings on gas turbine engine components are known to be susceptible to thinning from impact and erosion damage by hard particles in the high velocity gas path. Impact damage and the resulting loss of TBC particularly occur along the leading edges of components such as turbine blades, while erosion is more prevalent on the concave and convex surfaces of the blades, depending on the particular blade design. Both forms of mechanical damage not only shorten component life, but also lead to reduced engine performance and fuel efficiency.
Though mechanical damage such as erosion can be addressed by increasing the thickness of the TBC, a significant drawback is the additional mass added to the blade, resulting in higher centripetal loads that must be carried by a consequently heavier disk. Consequently, other solutions are necessary to achieve an impact and erosion-resistant TBC with an acceptable thickness, preferably less than 250 micrometers. Such attempts have included thermally treating the outer surface of the ceramic TBC material or providing an additional erosion-resistant outer coating. Suggested materials for more erosion-resistant outer coatings have included zircon, silica, chromia (Cr2O3) and alumina. While various methods and apparatuses are capable of sequentially depositing layers of different materials, a difficulty has been a tradeoff between spallation resistance and thermal conductivity. Spallation resistance is generally reduced by the presence of abrupt compositional changes at the interfaces between layers. On the other hand, and as discussed in U.S. Pat. No. 5,792,521 to Wortman, if the interfaces between layers are characterized by localized compositional gradients containing mixtures of the different deposited materials, the interface offers a poorer barrier to thermal conduction as compared to a distinct compositional interface in which minimal intermixing exists.
In view of the above, further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs.
The present invention generally provides a thermal barrier coating (TBC) for a component intended for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The TBC of this invention exhibits improved erosion and impact resistance as a result of containing a dispersion of alumina particles or precipitates (hereinafter referred to simply as particles). The TBC preferably consists essentially of yttria-stabilized zirconia and the alumina particles, which are dispersed throughout the microstructure of the TBC including the YSZ grains and grain boundaries. Importantly, the alumina particles are present in an amount sufficient to increase the impact and erosion resistance of the TBC, preferably at least 5 volume percent of the TBC.
In the form of discrete particles in the above-noted amount, sufficient alumina is present as a dispersion to increase the impact and erosion resistance of the TBC while avoiding the presence of localized compositional gradients that would decrease the spallation resistance of the TBC. The alumina particles serve to increase the fracture toughness of YSZ, and therefore the entire TBC, more effectively than a discrete layer of alumina at the TBC surface, particularly if the particles are dispersed throughout the TBC. The presence of alumina as discrete particles is also distinguishable from the prior art suggestion for using alumina in the form of discrete layers on individual YSZ grains of a TBC as a corrosion inhibitor. When present as a dispersion throughout the TBC (as opposed to discrete layers), the alumina particles provide uniform resistance to erosion and impact throughout the life of the TBC, including as the TBC erodes.
Suitable methods for depositing the TBC of this invention include plasma spraying and physical vapor deposition techniques. As an example, EBPVD can be used to deposit the TBC and its dispersion of alumina particles by evaporating multiple ingots, at least one of which is YSZ while a second contains alumina and optionally YSZ. In this method, the alumina content of the second ingot is continuously evaporated during the deposition process so that the alumina particles are dispersed throughout the TBC. Alternatively, the TBC can be deposited by evaporating a single ingot containing YSZ and regions of alumina. Another alternative is to evaporate a single ingot of YSZ using a chemical vapor deposition (CVD)-assisted process in which a source of aluminum vapors is continuously introduced into the coating chamber, causing oxidation of the aluminum and deposition of the resulting alumina vapors along with YSZ. Another method is to use an ion beam source of aluminum (cathodic arc source) while evaporating a YSZ ingot to create the dispersion of alumina particles in the YSZ TBC. With each of the alternative methods, the evaporation process is scalable to allow for the use of multiple coating sources.
The resulting TBC is characterized by improved resistance to both erosion and impact as a result of the alumina particles being present in sufficient amounts within the YSZ matrix of the TBC, and without being present as discrete layers on the YSZ grains or the surface of the TBC. As a result of improved erosion and impact resistance, relatively thinner TBC can be used as compared to conventional YSZ TBC to achieve the same service life. The net benefit is improved component life, engine performance and fuel efficiency.